Getting solid data on the consistency of cement
A
Magna Projects and Instruments product story
Edited by the Engineeringtalk editorial team
Jan 11, 2002
Magna Projects and Instruments has been licensed to develop and market a rugged machine for the Tattersall Two-Point Workability Test for fresh cement and concrete
Magna Projects and Instruments has been licensed by Heriot-Watt University and the University College of London (UCL) to develop and market the Tattersall Two-Point Workability Test (TTPT) for fresh cement and concrete.
This instrument, which was developed over the past 25 years of so at Sheffield University by Dr Geoff Tattersall, is designed to provided some solid engineering data on the consistency of mixes such as cement and concrete pastes.
Such data is vital for batch-to-batch quality control, and to ensure, for example, that unsuitable batches of material are not inadvertently poured onto such structures reinforced concrete buildings, thereby creating potential weak spots.
Cement and concrete pastes flow in a distinct way, behaving as "Bingham Fluids".
That is to say, they do not start to flow until a characteristic "Yield Stress" had been exceeded (a useful feature, as this allows the mix to stay in position until it sets), and then they flow in a linear or Newtonian manner, ie the shear stress (force) increases proportionally to the shear rate (speed).
Thus, the material may be characterised by the Yield Stress and by the slope of the ensuing straight line, which is, in fact, the plastic viscosity.
The TTPT is a rugged machine which may be used in the research and development or QA/QC laboratory or on site.
The principle of operation is to rotate a paddle in the sample - typically 20 - 30 litres - and to measure the torque required to rotate the paddle over range of speeds.
The torque vs speed curve then enables the Two Points to be evaluated.
The instrument may be operated in relative mode, or may be calibrated using materials of known rheology.
Two paddle geometries are available - a helical paddle, which rotates centrally, or an H-bladed paddle which rotates on a planetary gear for materials to avoid cavitation in susceptible formulations.
The instrument is fully guarded, and is automated so that it may be operated in constant speed, staircase or ramp speed modes.
Torque is measured by monitoring the oil pressure in the drive chain, and this is displayed on a panel-mounted pressure gauge and recorded by a computer via a pressure transducer.
Similarly, speed may be monitored and controlled by the computer, or may be changed manually.
Shear pins protect the drive against inadvertent overloads due, for example, to clumps of aggregate.
The TTPT may be hosed clean if required.
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